Page:First course in biology (IA firstcourseinbio00bailrich).pdf/136



a winter twig, seal the severed end with wax, and allow the twig to lie several days; it shrivels. There must be some upward movement of water even in winter, else plants would shrivel and die. 99. To illustrate sap pressure. The upward movement of sap water often takes place under considerable force. The cause of this force, known as root pressure, is not well understood. The pressure varies with different plants and under different conditions. To illustrate: cut off a strong-growing small plant near the ground. By means of a bit of rubber tube attach a glass tube with a bore of approximately the diameter of the stem. Pour in a little water. Observe the rise of the water due to the pressure from below (Fig. 128). Some plants yield a large amount of water under a pressure sufficient to raise a column several feet; others force out little, but under considerable pressure (less easily demonstrated). The vital processes (i.e., the life processes). 100. The pupil having studied roots, stems, and leaves, should now be able to describe the main vital functions of plants: what is the root function? stem function? leaf function? 101. What is meant by the "sap"? 102. Where and how does the plant secure its water? oxygen? carbon? hydrogen? nitrogen? sulfur? potassium?
 * ances, and the loss of weight will be noticed (Fig. 127). 98. Cut